Libraries

Overview

Teaching: 15 min
Exercises: 15 min

Questions

• How can I extend the capabilities of Python?

• How can I use software that other people have written?

• How can I find out what that software does?

Objectives

• Explain what software libraries are and why programmers create and use them.

• Write programs that import and use libraries from Python’s standard library.

• Find and read documentation for standard libraries interactively (in the interpreter) and online.

Most of the power of a programming language is in its (software) libraries.

• A (software) library is a collection of files (called modules) that contains functions for use by other programs.
  • May also contain data values (e.g., numerical constants) and other things.
  • Library’s contents are supposed to be related, but there’s no way to enforce that.
• The Python standard library is an extensive suite of modules that comes with Python itself.
• Many additional libraries are available from PyPI (the Python Package Index).
• We will see later how to write new libraries.

Libraries and modules

A library is a collection of modules, but the terms are often used interchangeably, especially since many libraries only consist of a single module, so don’t worry if you mix them.

A program must import a library module before using it.

• Use import to load a library module into a program’s memory.
• Then refer to things from the module as module_name.thing_name.
  • Python uses . to mean “part of”.
• Using string, one of the modules in the standard library:

import string

print('The lower ascii letters are', string.ascii_lowercase)
print(string.capwords('capitalise this sentence please.'))

The lower ascii letters are abcdefghijklmnopqrstuvwxyz
Capitalise This Sentence Please.

• You have to refer to each item with the module’s name.
  • string.capwords(ascii_lowercase) won’t work: the reference to ascii_lowercase doesn’t somehow “inherit” the function’s reference to string.

Use help to learn about the contents of a library module.

• Works just like help for a function.

help(string)

Help on module string:

NAME
    string - A collection of string constants.

MODULE REFERENCE
    https://docs.python.org/3.6/library/string
    
    The following documentation is automatically generated from the Python
    source files.  It may be incomplete, incorrect or include features that
    are considered implementation detail and may vary between Python
    implementations.  When in doubt, consult the module reference at the
    location listed above.

DESCRIPTION
    Public module variables:
    
    whitespace -- a string containing all ASCII whitespace
    ascii_lowercase -- a string containing all ASCII lowercase letters
    ascii_uppercase -- a string containing all ASCII uppercase letters
    ascii_letters -- a string containing all ASCII letters
    digits -- a string containing all ASCII decimal digits
    hexdigits -- a string containing all ASCII hexadecimal digits
    octdigits -- a string containing all ASCII octal digits
    punctuation -- a string containing all ASCII punctuation characters
    printable -- a string containing all ASCII characters considered printable

CLASSES
    builtins.object
        Formatter
        Template
⋮ ⋮ ⋮

Create an alias for a library module when importing it to shorten programs.

• Use import ... as ... to give a library a short alias while importing it.
• Then refer to items in the library using that shortened name.

import string as s

print(s.capwords('capitalise this sentence again please.'))

Capitalise This Sentence Again Please.

• Commonly used for libraries that are frequently used or have long names.
  • E.g., The pandas library is often aliased as pd.
• But can make programs harder to understand, since readers must learn your program’s aliases.

Import specific items from a library module to shorten programs.

• Use from ... import ... to load only specific items from a library module.
• Then refer to them directly without library name as prefix.

from string import ascii_letters

print('The ASCII letters are', ascii_letters)

The ASCII letters are abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ

When Is Help Available?

When a colleague of yours types help(os), Python reports an error:

NameError: name 'os' is not defined

What has your colleague forgotten to do?

Solution

Importing the os module (import os)

Exploring the os Library

The os library provides a way of accessing operating system functionality.

1. What function from the os library can you use to determine the current working directory?

Solution

1. Using help(os) we see that we’ve got os.getcwd() which returns a string representing the current working directory.

There Are Many Ways To Import Libraries!

Match the following print statements with the appropriate library calls

Library calls:

A) from string import digits
B) import string
C) import string as s

Print commands:

1. print(list(s.digits))
2. print(list(digits))
3. print(string.ascii_uppercase)

Solution

A2) Importing digits from string provides the digits methods B3) Importing string provides methods such as ascii_uppercase, but requires the string. syntax. C1) Importing string with the alias s allows s.digits

Reading Error Messages

1. Read the code below and try to identify what the errors are without running it.
2. Run the code, and read the error message. What type of error is it?

import datetime
datetime.date(2017,13,1)

Solution

1. The date object takes arguments in the order year, month, day, so 13 is an invalid value for month.
2. You get an error of type “ValueError”, indicating that the object received an inappropriate argument value. The additional message “month must be in 1..12” makes it clearer what the problem is.

Locating the Right Module

Given the variables year, month and day, how would you generate a date in the standard iso format:

year = 2016
month = 10
day = 22

1. Which standard library module could help you?
2. Which function would you select from that module?
3. Try to write a program that uses the function.

Solution

The datetime module seems like it could help you.

You could use date(year, month, date).isoformat() to convert your date:

import datetime

iso_date = datetime.date(year, month, day).isoformat()
print(iso_date)

or more compactly:

import datetime

print(datetime.date(year, month, day).isoformat())

Importing With Aliases

1. Fill in the blanks so that the program below prints 0123456789.
2. Rewrite the program so that it uses import without as.
3. Which form do you find easier to read?

import string as s
numbers = ____.digits
print(____)

Solution

import string as s
numbers = s.digits
print(numbers)

can be written as

import string
numbers = string.digits
print(numbers)

Since you just wrote the code and are familiar with it, you might actually find the first version easier to read. But when trying to read a huge piece of code written by someone else, or when getting back to your own huge piece of code after several months, non-abbreviated names are often easier, expect where there are clear abbreviation conventions.

Importing Specific Items

1. Fill in the blanks so that the program below prints 90.0.
2. Do you find this version easier to read than preceding ones?
3. Why wouldn’t programmers always use this form of import?

____ math import ____, ____
angle = degrees(pi / 2)
print(angle)

Solution

from math import degrees, pi
angle = degrees(pi / 2)
print(angle)

Most likely you find this version easier to read since it’s less dense. The main reason not to use this form of import is to avoid name clashes. For instance, you wouldn’t import degrees this way if you also wanted to use the name degrees for a variable or function of your own. Or if you were to also import a function named degrees from another library.

Key Points

• Most of the power of a programming language is in its libraries.

• A program must import a library module in order to use it.

• Use help to learn about the contents of a library module.

• Import specific items from a library to shorten programs.

• Create an alias for a library when importing it to shorten programs.